Magnet wire is small diameter copper or aluminum wire coated with an insulating material known as wire enamel. The wire enamel usually is composed of a base resin and various additives which improve the properties and performance of the overall composition.
In the present electrical and electronics industry, large amounts of magnet wire are used in manufacturing motors, transformers, television yoke coils and many other products. The design of electrical equipment and machinery has required progressively smaller motors, coils, transformers and other products using magnet wire, and these operate at higher temperatures than larger structures. Therefore, the demand for higher temperature resistant magnet wire used in these electrical motors and transformers has been increasing. For present purposes, temperature resistance is defined in terms of the thermal index temperature measured according to ASTM D-2307 and/or NEMA (National Electrical Manufacturers Association) ratings.
A further requirement for such products, for many applications, is solderability of the free ends of magnet wire extending from such structures. It is important that the wire be capable of being soldered quickly.
In many areas of the industry, there is increased demand for precise quality control of products. This demand is leading the industry to turn to automated production. In the electrical and electronics industry, an automated production system has a significant place in whatever the product is. In handling these products where magnet wire is used, the questions of solderability temperature and speed of solderability of the magnet wire may determine how successfully an automated production line can be implemented.
Many types of wire enamel are known. These include conventional high-temperature enamels such as those in which the base resin is a polyimide, polyamideimide, polyesterimide (or polyesteramideimide) or polyester. There are two main types of polyesterimide wire enamels, and these differ in thermal classification. One type is a polyesterimide in which there are polyester units derived from polyfunctional aliphatic alcohols such as glycerine and trimethylolpropane as well as diols such as ethylene glycol. The polyester units also contain aromatic carboxylic acids, usually terephthalic acid. In the second type, the polyester units contain an isocyanurate group, normally the group derived from tris-(2-hydroxyethyl) isocyanurate (THEIC), in place of the polyfunctional aliphatic alcohol. In both cases, the polyesterimide contains imide units derived normally from an aromatic diamine, such as methylenedianiline, and a polyfunctional carboxylic acid or acid/anhydride (normally trimellitic anhydride).
The type of polyesterimide which contains aliphatic polyols is usually classified as having a thermal resistance less than Class 180 according to the NEMA standard. The type of polyesterimide which contains isocyanurate rings in the molecule normally is classified as Class 180 or higher according to the NEMA standard. The latter type of polyesterimide wire enamel is described in U.S. Pat. No. 3,426,098.
Likewise, it is also known that there are two different types of polyester wire enamels. One type is based on polyester in which there are polyfunctional aliphatic alcohols but no isocyanurate rings. Wire enamels of this type are generally rated as Class 155 according to the NEMA standard. The other type is based on a polyester in which there are polyol units which contain an isocyanurate ring. These wire enamels normally are rated as over Class 180 according to the NEMA standard. This type of polyester is described in U.S. Pat. No. 3,342,780.
Whether they contain an isocyanurate ring in the molecule or not, polyester wire enamels normally require nylon or polyamideimide topcoat for the majority of their applications.
Among the base resins mentioned above, the only resin which can provide a reasonably solderable high-temperature resistant magnet wire enamel is polyesterimide not containing an isocyanurate ring in the molecule. However, in order to make the material solderable, a substantial reduction in the molecular weight is required in this type of polyesterimide. Unfortunately, this reduction in molecular weight of the base resin leads to a deterioration of the high-temperature resistance properties of the wire enamel. Furthermore, even when the molecular weight of the polyesterimide is reduced, the solderability of these wire enamels is still insufficient in terms of speed of soldering and the low temperature needed for soldering.
The only base resin which provides wire meeting the requirement for fast-speed and low-temperature soldering is conventional polyurethane. Polyurethanes are polymers produced by reaction of a polyfunctional isocyanate and a prepolymer containing free hydroxyl groups. The free hydroxyl groups may be provided, for example, by polyesters or polyethers. Such a polyurethane resin is shown in U.S. Pat. No.3,174,950 wherein methylene bisphenylisocyanate is reacted with tris (2-hydroxyethyl) isocyanurate (THEIC). However, wire enamels based on this type of polymer cannot provide sufficient high-temperature resistance to meet the requirements of the Class F and Class H standards set by NEMA.
U.S. Pat. No. 3,869,428 describes a polyurethane enamel composition wherein the composition contains imide and urethane groups and is the reaction product of an aromatic hydroxyl group-containing compound and an aromatic isocyanate, wherein one of these reactants contains imide groups. The composition according to U.S. Pat. No. 3,869,428 is said to result in solderability comparable with polyurethane enamels and temperature resistance properties comparable to polyesterimide enamels.